###
A list represents an ordered collection of items.

Since Yack is totally dynamically typed, lists can be pretty
free-form. They don't need to contain a consistent type of
element, and they don't even necessarily need a terminating
{Nil()} value (though that's still recommended...)

@example#empty
  Nil()
@end

@example#oneTwoThree
  Cons(1, Cons(2, Cons(3, Nil())))
@end
###
data List = Nil() | Cons(head, tail)

###
This returns the first element of a {List#}, or {void} if
the list is empty.
###
def head = fun(list)
  match list
  | Cons(head, _) -> head
  | _ -> void
  end
end

###
Returns everything BUT the first element of a {List#}, or
{Nil()} if the list is already empty.
###
def tail = fun(list)
  match list
  | Cons(_, tail) -> tail
  | _ -> List#empty
  end
end

###
Reverses the given {List#}, using cute recursion.

For example, if we reverse {List#oneTwoThree}:
@example#oneTwoThree
  reverse(List#oneTwoThree)
@end
We get {reverse#oneTwoThree}
###
def reverse = fun(list)
  (fun reverse(list, reversed)
    match list
    | Nil() -> reversed
    | Cons(head, tail) ->
      reverse(tail, Cons(head, reversed))
    end
  end)(list, Nil())
end